nail popping at plywood underlaymen

huangyhg

nail popping at plywood underlayment
a restaurant we designed about a year ago is experiencing a problem.  the 5/8" thick plywood underlayment was nailed to the 5/8" plywood floor sheathing with 8d "ringshank" nails at 12" oc.  at the bar area, where ceramic tile occurs over the underlayment, nails are backing out of the underlayment and causing bulges in the ceramic tile.
what is causing this?  some ceramic tile was removed, and no evidence of moisture damage was found in the plywood.  the contractor thinks it is being caused by patrons jumping in and out of bar stools, causing the floor to bounce.  but we did use tji floor joists.
never had much faith in ringshank nails, i'm not a contractor but i've always made sure that nails for that application were actually screws, hope someone else has a better answer...mike
dave,
i agree with mike that ring shanks don't always work well.  i suspect you are having a combination of issues, the most prominent being wet-dry cycling of the plywood from the cleaning process.  this occurs without the presence of explicit moisture damage showing in the plywood.  it is a result of very small volumetric changes in the plywood, but they lift the head of the nail initially, then a similar process works on the shank.  this is a non-recoverable action in that the nails go up, but they don't go back down, 'cause there's no action to cause them to do so!  this results in the nails backing out.
while the bouncing might contribute after a while, it is not likely a big part of the problem.
solution is to use corrosion resistant screws, spaced at about 6 to 8 inches at every support.
american plywood assn approved construction adhesive between the joists and plywood would have been a good preventative measure in original construction.